S. P. Zhao

Quantum phase diffusion in a small underdamped Josephson junction.

Quantum phase diffusion in a small underdamped Nb/AlO(x)/Nb junction (∼0.4 μm(2)) is demonstrated in a wide temperature range of 25-140 mK where macroscopic quantum tunneling (MQT) is the dominant escape mechanism. We propose a two-step transition model to describe the switching process in which the escape rate out of the potential well and the transition rate from phase diffusion to the running state are considered. The transition rate extracted from the experimental switching current distribution follows the predicted Arrhenius law in the thermal regime but is greatly enhanced when MQT becomes dominant.

A cryogen-free dilution refrigerator based Josephson qubit measurement system

We develop a small-signal measurement system on cryogen-free dilution refrigerator which is suitable for superconducting qubit studies. Cryogen-free refrigerators have several advantages such as less manpower for system operation and large sample space for experiment, but concern remains about whether the noise introduced by the coldhead can be made sufficiently low. In this work, we demonstrate some effective approaches of acoustic isolation to reduce the noise impact. The electronic circuit that includes the current, voltage, and microwave lines for qubit coherent state measurement is described. For the current and voltage lines designed to have a low pass of dc-100 kHz, we show that the measurements of Josephson junctions switching current distribution with a width down to 1 nA, and quantum coherent Rabi oscillation and Ramsey interference of the superconducting qubit can be successfully performed.

Crossover behaviors in magnetoresistance oscillations for Nb thin film with rectangular arrays of antidots

Superconducting Nb thin films with rectangular arrays of submicron antidots (holes) have been systemically investigated by transport measurements. A series of crossover behaviors is found in magnetoresistance oscillations, corresponding to three different superconducting states: the wire network-like state, the interstitial vortex state and the single-loop–like state. These states are identified by the field intervals and hysteretic effect. The crossover fields between them are found to be both temperature and geometry dependent. Furthermore, in dense arrays, the saturation number is distinctly larger than the theoretical calculation for a single insulating inclusion. Our results indicate that in the process of magnetic field penetration into nanostructured superconductors, the order parameters are strongly modulated and finally localized near the edges, resulting in changes of oscillation modes.

Fabrication of Nb/Al2O3/Nb Josephson Junctions Using In Situ Magnetron Sputtering and Atomic Layer Deposition

Atomic layer deposition (ALD) provides a promising approach for deposition of ultrathin low-defect-density tunnel barriers, and it has been implemented in a high-vacuum magnetron sputtering system for in situ deposition of ALD-Al₂O₃ tunnel barriers in superconductor-insulator-superconductor Josephson junctions. A smooth ALD-Al₂O₃ barrier layer was grown on an Al-wetted Nb bottom electrode and was followed with a top Nb electrode growth using sputtering. Preliminary low temperature measurements of current-voltage characteristics of the Josephson junctions made from these trilayers confirmed the integrity of the ALD-Al₂O₃ barrier layer. However, the <i>I</i>_c<i>R</i>_N product of the junctions is much smaller than the value expected from the Ambegaokar-Baratoff formula suggesting a significant pair-breaking mechanism at the interfaces.

Wire network behavior in superconducting Nb films with diluted triangular arrays of holes

We present results of transport measurements on superconducting Nb films with diluted triangular arrays (honeycomb and kagomé) of holes. The patterned films have large disk-shaped interstitial regions even when the edge-to-edge separations between nearest neighboring holes are comparable to the coherence length. Changes in the field interval of two consecutive minima in the field dependent resistance R(H) curves are observed. In the low field region, fine structures in the R(H) and T(c)(H) curves are identified in both arrays. Comparison of experimental data with calculation results reveals that these structures observed in honeycomb and kagomé hole arrays resemble those in wire networks with triangular and T(3) symmetries, respectively. The findings suggest that even in these specified periodic hole arrays with very large interstitial regions, the low field fine structures are determined by the connectivity of the nanostructures.

Quantum and classical resonant escapes of a strongly driven Josephson junction

The properties of phase escape in a dc superconducting quantum interference device (SQUID) at 25 mK, which is well below quantum-to-classical crossover temperature T(cr), in the presence of strong resonant ac driving have been investigated. The SQUID contains two Nb/Al-AlO(x)/Nb tunnel junctions with Josephson inductance much larger than the loop inductance so it can be viewed as a single junction having adjustable critical current. We find that with increasing microwave power W and at certain frequencies nu and nu/2, the single primary peak in the switching current distribution, which is the result of macroscopic quantum tunneling of the phase across the junction, first shifts toward lower bias current I and then a resonant peak develops. These results are explained by quantum resonant phase escape involving single and two photons with microwave-suppressed potential barrier. As W further increases, the primary peak gradually disappears and the resonant peak grows into a single one while shifting further to lower I. At certain W, a second resonant peak appears, which can locate at very low I depending on the value of nu. Analysis based on the classical equation of motion shows that such resonant peak can arise from the resonant escape of the phase particle with extremely large oscillation amplitude resulting from bifurcation of the nonlinear system. Our experimental result and theoretical analysis demonstrate that at T << T(cr), escape of the phase particle could be dominated by classical process, such as dynamical bifurcation of nonlinear systems under strong ac driving.

Fabrication of

Atomic layer deposition (ALD) provides a promising approach for deposition of ultrathin low-defect-density tunnel barriers, and it has been implemented in a high-vacuum magnetron sputtering system for in situ deposition of ALD-Al₂O₃ tunnel barriers in superconductor-insulator-superconductor Josephson junctions. A smooth ALD-Al₂O₃ barrier layer was grown on an Al-wetted Nb bottom electrode and was followed with a top Nb electrode growth using sputtering. Preliminary low temperature measurements of current-voltage characteristics of the Josephson junctions made from these trilayers confirmed the integrity of the ALD-Al₂O₃ barrier layer. However, the <i>I</i>_c<i>R</i>_N product of the junctions is much smaller than the value expected from the Ambegaokar-Baratoff formula suggesting a significant pair-breaking mechanism at the interfaces.

\hbox{Nb/Al}_{2}\hbox{O}_{3}/\hbox{Nb}

Three different Pb films-free standing, on a semiconducting Ge(111) substrate, and on a metallic Cu(111) substrate-are studied with first-principles calculations. Our studies show that the properties of these films-surface energy, work function, and lattice relaxation-oscillate strongly with the film thickness. The oscillation follows a bilayer pattern interrupted by even-odd crossovers. However, the positions of the crossovers and the separation between the crossovers depend on the substrate, showing that the substrate plays an important role in the Pb film properties. In particular, the results for Pb films on Cu(111) substrate challenge the existing physical picture of Pb films.

Josephson Junctions Using In Situ Magnetron Sputtering and Atomic Layer Deposition

The SQUID microscope is the most suitable instrument for imaging magnetic fields above sample surfaces if one is mainly interested in field sensitivity. In this paper, both the magnetic moment sensitivity and spatial resolution of the SQUID microscope are analysed with a simple point moment model. The result shows that the ratio of SQUID sensor size to sensor–sample distance effectively influences the sensitivity and spatial resolution. In comparison with some experimental results of magnetic images for room temperature samples from our high-Tc SQUID microscope in an unshielded environment, a brief discussion for further improvement is presented.

Reversible magnetization and critical fluctuations in systematically doped YBa2Cu3O7-delta single crystals

Metal-Insulator-Metal tunnel junctions (MIMTJ) are common throughout the microelectronics industry. The industry standard AlOx tunnel barrier, formed through oxygen diffusion into an Al wetting layer, is plagued by internal defects and pinholes which prevent the realization of atomically-thin barriers demanded for enhanced quantum coherence. In this work, we employed in situ scanning tunneling spectroscopy (STS) along with molecular dynamics simulations to understand and control the growth of atomically thin Al2O3 tunnel barriers using atomic layer deposition (ALD). We found that a carefully tuned initial H2O pulse hydroxylated the Al surface and enabled the creation of an atomically-thin Al2O3 tunnel barrier with a high quality M-I interface and a significantly enhanced barrier height compared to thermal AlOx. These properties, corroborated by fabricated Josephson Junctions, show that ALD Al2O3 is a dense, leak-free tunnel barrier with a low defect density which can be a key component for the next-generation of MIMTJs.